Clarified Slurry Oil, (CSO), a heavy oil produced as a byproduct of catalytic cracking is an important commercial product. These highly aromatic, high boiling, dense liquids are the hydrocarbon fractions which remain as a bottoms fraction after catalytic cracking.
These materials are primarily used as heavy fuel oil, and to a lesser extent as charge stock for conversion to carbon black or used as aromatic solvents. In many of the downstream uses of these heavy oils, it is important that the oil be free of particulates. Presence of even minor amounts of particulates, such as cracking catalyst "fines", can make the heavy fuel oil totally unsuitable for use in internal combustion engines. The cracking catalyst fines are highly abrasive and difficult to remove by conventional filtering because of their extremely fine size. Fines accumulate in the pistons of the internal combustion engines burning the fuel, so the presence of even small amounts of fines in heavy fuel oil cannot be tolerated.
The problem can be better understood by considering what happens to heavy oil feed in a catalytic cracking unit. Fluidized catalytic cracking (FCC) units are the most popular so the conversion of heavy feed to lighter products in an FCC unit will be described. Similar problems occur in Thermofor catalytic cracking (TCC) units, though the problems are not as severe as in FCC.
Hot, regenerated FCC catalyst contacts the heavy feed in the base of a reactor. The catalyst is present in excess, usually 5 or 10 weights of catalyst per weight of oil. The FCC catalyst has a particle size distribution ranging from 10-100 microns. The FCC catalyst cracks the oil to produce lighter products and is coked in the process. Coked catalyst is separated from cracked products. The coked catalyst is regenerated in a fluidized bed combustion zone or regenerator. The cracked products and entrained catalyst fines are sent to a main fractionation column which separates cracked products into a range of products, gaseous fractions, naphtha, light fuel oil and a bottoms fraction.
Because of the nature of catalytic cracking units, there is always a significant amount of catalyst entrained with the cracked products. Fines are created continuously in the FCC unit, primarily by particles of catalyst bumping into one another, or hitting hard surfaces in the unit. A commercial FCC unit, with 100 tons of equilibrium catalyst, might generate 1000 to 2000 pounds per day of fines. The main fractionation column is designed to accomodate the fines, and any fines carried over into the main column are collected with the bottoms fraction. The bottoms fraction is frequently recycled, at least in part, to the FCC reactor for additional conversion. This recycle sends more catalyst fines back to the FCC reactor, which increases the catalyst fines content of cracked products.
As a practical matter, it is usually not possible to recycle 100% of the material withdrawn from the main column bottoms (MCB). This material is highly aromatic and difficult to crack. Usually the MCB is sent to a slurry settler. Most of the fines settle to the bottom. The oil withdrawn from the bottom of the slurry settler is the slurry phase. The oil withdrawn above the slurry phase is clarified slurry oil. The slurry phase is recycled, and the clarified slurry oil (CSO) withdrawn and sent to a storage tank. Some cooled CSO may be recylced to the slurry settler. During storage, more of the catalyst fines settle to the bottom of the tank leaving an oil phase with a further reduced fines content. However, the settling is slow and, settling never removes all of the catalyst particles. Slurry settling merely removes "fines" that are fairly large, and easy to recover by settling. The finer catalyst particles are removed only with great difficulty by such settling, and the CSO (still with a small but significant amount of extremely fine catalyst fines remaining) is withdrawn and used as a product.
Filtering of the CSO should remove more of the fines and make the product completely suitable for fuel oi, but the particles remaining in the CSO are extremely fine so conventional filtration methods are not as effective as desired in removing this material. The filtration is extremely difficult because the CSO is a dense viscous liquid, and the particulate contaminants to be removed by filtration consist almost exclusively of particles which are so fine that they have resisted hours of high temperature settling in a slurry settler, and in some cases, days of settling tank in a tank.
Some use has been made of slurry oils in FCC. In U.S. Pat. No. 4,264,428 (Schoennagel et al), incorporated herein by reference, a coal liquification process used FCC main column bottoms in a coal solubilization zone. The resulting coal/solvent mixture was then charged to the base of an FCC riser reactor. The process would not reduce the catalyst fines contents of the FCC main column bottoms stream, nor of clarified slurry oil derived therefrom. The solids content of the CSO stream from the coal solubilization zone would increase.
We reviewed the existing ways of preparing a CSO of reduced fines content and could find no completely satisfactory and economical solution to the problem.
We realized conventional methods could not work too well because they allowed the fines to accumulate or build up in the FCC. They also allowed the oil to cool too much, both in the slurry settler and in storage. Once these two conditioners were met, large fines content remaining in the oil and lower temperature oil with increased viscosity, both settling and filtration became difficult.